The Odds of Getting Hit by an Asteroid

Astronomy: Scientists approve method of calculating, on a scale of 1 to 10, the probability of a collision with Earth.

Much like earthquakes are assigned their familiar magnitude ratings, asteroids now have their own Richter scale.

Planetary scientist Richard Binzel of MIT unveiled his Torino Impact Hazard Scale on Tuesday to a meeting of astronomers at Cornell University. Most scientists applauded the scale, citing last year's reaction to asteroid 1997XF11--which some initially said could collide with Earth in 2028--as an example of communication gone wrong (such predictions were quickly contradicted by further measurements).

The Torino scale--dubbed "Richter" by Binzel and others after the original earthquake scale--will show exactly how much of a threat an asteroid poses to Earth, proponents say.

A zero on the Torino scale says that a collision is impossible; a 10 indicates a certain collision with an asteroid that is large enough--probably more than half a mile across--to cause global catastrophe.

All currently identified asteroids have a zero rating. However, only a small fraction of the estimated 2,000 large asteroids that could come near Earth have been found, according to David Morrison, director of astrobiology and space research at NASA's Ames Research Center in Mountain View, Calif. With so many left to identify, scientists are increasing their efforts in scanning the skies.

"The topic is so provocative, [we realized] we have to take great care," Binzel said. "And the time was coming when discovery of close approaching asteroids would become routine."

Binzel first conceived the scale in 1994 and proposed it to a group of astronomers in 1995; they chose not to adopt it. He made some changes and believes that now the time is right. The scale was endorsed by the International Astronomical Union on July 22.

"We have had actual cases of newly discovered objects for which a collision [initially] could not be ruled out," he said. "And so we have found that we need a clear and efficient way of bringing objects to public attention that doesn't cause unnecessary alarm."

For example, he thinks the XF11 threat would have been put in perspective. "Pretend [the Torino scale] existed," he says. "XF11 would have been placed as a 1," indicating that, while the asteroid should be monitored, there was no cause for concern.

Nobody, he thinks, would have lost sleep over an earthquake rated 1 on the old Richter scale. As astronomers got a better idea of the asteroid's likely path, "XF11 would have been reassigned to zero." No panic, no problem.

Instead, when a news release said that the collision couldn't be ruled out, some headlines boomed: "Could This Be the Deadline for the Apocalypse?"; "Asteroid Seen on Deadly Course for 2028"; and "Is This the End of the World?"

When earlier locations of XF11 were identified and added to the orbit calculations, astronomers announced that XF11 would pass by with room to spare. Some researchers think they lost credibility over the incident.

"Astronomers, myself included, looked bad because we had to revise our calculations," said Paul Chodas, a planetary scientist at Jet Propulsion Laboratory.

The scale itself will not save scientists from revisions--no Torino number will be issued until calculations have been checked by other scientists, said Donald Yeomans, manager of the Near-Earth Objects project at JPL. But it will offer the promise of unprecedented accuracy in a prediction.

Joe Montani, a senior research technician at the University of Arizona's Spacewatch program, is concerned about the time it will take scientists to issue a number. He says he "wants to know everything about any object that is discovered" as soon as possible, rather than having to wait for an official, unified proclamation.

Astronomers estimate that a large asteroid hits the Earth every million years or so. Earth is a big stake to bet on averages, though, so scientists have been intensifying the study of individual asteroids and their likelihood of earthly collisions.

"If you decide to cross a street at a busy intersection, you may have an intellectual interest in the statistics of pedestrians being run down," Morrison said, "but you really just want to know if you personally are going to get hit."

What if researchers find an asteroid that poses a real threat?

"The odds of seeing something higher than 1 are very small, but the numbers are there and ready to be used," said Chodas.

By looking skyward, earthlings are increasing the chances of finding such a space rock many years before a collision would occur, said Morrison.

"This probably won't be a Hollywood scenario," he said. "We'll probably have lots of warning. Unlike other natural hazards, we could do something about it.

"Obviously, we need to think about what we would do [if we found a dangerous asteroid]. Whether we should spend money or resources on [combating] it ahead of time, when the likelihood is so low, is not so obvious."